Nitrate in drinking water systems usually originates from fertilizers or from animal or human wastes. Nitrate concentrations in the natural water system tend to be highest in areas of intensive agriculture or where there are many septic systems. High nitrogen and phosphorus content in the water body has impeded water reuse potential and impacted ecosystem integrity and human health. Nitrate (NO3−) can potentially be toxic and can cause human health problem such as methemoglobinemia, liver damage and possible cancers according to a World Health Organization, 2006 report. Phosphorus can potentially trigger the eutrophication issues in fresh water bodies, which could result in toxic algae and endanger the source of drinking waters eventually (ESA, 2000).
When urban regions gradually expand due to regional development, centralized sewage collection, treatment, and disposal is often unavailable for both geographic and economic reasons. Thus, decentralized or on-site wastewater treatment systems (OWTS) may be necessary to protect public health. Nationwide, wastewater effluent from on-site wastewater treatment systems can represent a large fraction of nutrient loads to groundwater aquifers.
In the modern era, on-site wastewater treatment systems also referred as septic system primarily includes a septic tank and soil adsorption field or drainfield also known as subsurface wastewater infiltration systems. Drainfields are located in permeable, unsaturated natural soil or imported fill material so wastewater can infiltrate and percolate through the underlying soil to the ground water thereby treating itself through a variety of physical, chemical, and microbiological processes. However, the nitrate ion (NO3−) and soil are negatively charged, and so the NO3− ion is not bound to the soil. Therefore, nitrate ions move freely with the soil solution and are readily leachable. Nitrogen, particularly nitrates, easily moves from terrestrial ecosystems into surface and groundwater, including lakes, streams, rivers, and estuaries as described in Peterson et al., Symptoms of nitrogen saturation in two central Appalachian hardwood forest ecosystems. Biogeochemistry 35, pp. 507-522 (1996).
Two important processes that result in the transformation of nitrate are nitrification and dentrification. Nitrification is a process in which ammonium is oxidized and denitrification is a process in which nitrate is reduced back to nitrogen gas before escaping into the air. However, only denitrification that is a microbiologically mediated process occurring under anaerobic (oxygen depleted) conditions can result in the permanent removal of nitrate. Approximately, 55-85% of the nitrogen that enters the septic tank is available to ground water mainly in the form of nitrates as described in Stoltz and Reneau, Potential for Contamination of Ground and Surface Waters from On-site Wastewater Disposal Systems (1998). Based on recent Florida research data, a family of four discharges approximately 11.36 Kg (25 pounds) of nitrogen (measured in the form of nitrates) per year into the drainfield of a conventional onsite sewage treatment and disposal system according to a Florida Department of Health report dated 2004.
The main risks of nitrates pollutants are in “Blue baby” syndrome and suspected carcinogenic effect of nitrates on humans, and the nutrient enrichment of receiving waters. It has regulatory health limits in the United States of maximum contamination level (MCL) of 10 mg-N L−1. A septic tank with a conventional drainfield does not typically remove nitrogen in the form of nitrates since it is very soluble and does not sorb well to soil components during infiltration.
The use of different sorption media in septic tank drainfields turns out to be an appealing engineering approach in dealing with the increasing trend of higher nitrate concentrations that is expected to continue in the surface and groundwater systems. Besides, the use of the sorption media for denitrification rather than traditional gravel-filled drainfield for handling the effluents from the septic tank system would become a new focus in rural communities. Large-scale implementation with different sorption media to remove nutrients will be popular in the future. See Mothersill, C. L., Anderson, B. C., Watt, W. E., and Marsalek, J., Biological filtration of stormwater: field operations and maintenance experiences and Birch, G. F., Fuseli, M. S., and Matthias, C., Efficiency of an infiltration basin in removing contaminants from urban stormwater. Environmental Monitoring and Assessment, 101, pp. 23-38, (2005).
It is believed that functionalized sorption media might have a better ion exchange capacity to support adsorption/desorption capacity. Research that lead to the subject matter of the present invention included screening sorption media via a thorough literature review, characterization of the selected sorption media, and examination of their sorption capacity for nutrient removal using column study, isotherm tests and microcosm assessment in support of the new underground drainfield design as an integral part of modern septic tank system.
Many researchers had tried to remove nitrogen species by using sorption media. Kim, H., Seagren, E. A., and Davis A. P., Engineering Bioretention for Removal of Nitrate from Storm water Runoff, in WEFTEC 2000 Conference Proceedings on CDROM Research Symposium, Nitrogen Removal, Session 19, Anaheim Calif., October (2000) used different kinds of sorption media, such as alfalfa, mulch compost, newspaper, sawdust, wheat straw, wood chips for nitrate removal from storm water runoff. They found that alfalfa and newspaper had 100% nitrate removal efficiency but mulch compost had 60% nitrate removal efficiency. They also found that sawdust, wheat straw and wood chips had good removal efficiency greater than 95% but wood chips showed consistently better performance in nitrate removal over sawdust. From their experiment, it could be concluded that all of these were electron donors and good carbon sources for promoting denitrification. They suggested that increasing the retention time may gain better removal efficiency. Kim et al. also found that soil could only remove 7% to 10% of nitrate due to its anionic form.
Güngör, K. and Ünlü, K., 2005. Nitrite and nitrate removal efficiencies of soil aquifer treatment columns, Turkish J. Eng. Env. Sci., 29, pp. 159-170, (2005) conducted nitrate and nitrite removal experiment by using only three types of soils, including sandy clay loam, loamy sand and sandy loam. They found significant nitrate and nitrite removal (i.e., over 90%). Hsieh, C. H. and Davis, A. P., Multiple-event study of bioretention for treatment of urban storm water runoff, Diffuse Pollution Conference Dublin, Ireland (2005) found that mulch was very effective in removing nitrate, unlike sand. But they had not gained good ammonia removal efficiency by using mulch. They concluded that soil with higher silt/clay and cation (Mg/Ca/K) contents might be very effective in nutrient removal. They also concluded that course media might not be able to retain the nutrient in repetitive loading due to small surface area so that sand should not be used.
Darbi, A., Viraraghavan, T., Butler, R., and Corkal, D., Batch studies on nitrate removal from potable water, Water South Africa, 28(3), pp. 319-322, (2002) used sulfur and limestone for nitrate removal from potable water. In their experiment, sulfur was used as an electron donor and limestone was used to maintain the pH. They found that the optimum mixing ratio of sulfur and limestone is 1:1 for nitrate removal (i.e., about 98% nitrate removal was observed). They also suggested that increasing the retention time may obtain higher nitrate removal efficiency. Eisi, R.D., Park, J. K., and Stier, J. C., Mitigating Nutrient Leaching with a Sub-Surface Drainage Layer Of Granulated Tires. Waste Management, 24(8), pp. 831-839, (2004) tried to use granulated tire for the removal of nitrate. They found 48.000 g of tire crumb can remove 16.2 g of NO3−—N. Sengupta, S. and Ergas, S. J., describe Autotrophic biological denitrification with elemental sulfur or hydrogen for complete removal of nitrate-nitrogen from a septic system wastewater.
The NOAA/UNH cooperative institute for costal and estuarine environmental technology (2006) did an experiment to remove nitrate from wastewater by using marble chips, limestone and oyster shell. Their experiment gave some significant outcomes about using those solids as sorption media. They found that oyster shell containing almost 98% CaCO3 and limestone could remove 80% and 56% of nitrate, respectively. The pH and alkalinity were higher in testing using oyster shell rather than limestone and marble chips. Oyster shell was efficient to reduce nitrite accumulation and dissolved oxygen did not work as a denitrification inhibitor when oyster shell was used as a sorption media. From these findings, it can be concluded that oyster shell is much more effective than limestone or marble chips for removing nitrate. Oyster shell can also be a good candidate for controlling the pH that is sensitive for denitrification.
Savage A. J. and Tyrrel, S. F., 2005. Compost liquor bioremediation using waste materials as biofiltration media, Bioresource Technology, 96, pp. 557-564 (2005) used wood mulch, compost, soil, broken brick and polystyrene packaging for removal of NH3—N from compost leachate. They reached a conclusion that wood mulch (75%) and compost (55%) had better removal efficiency for NH3—N that other media and polystyrene was the least capable one to remove NH3—N. Soil and broken brick could remove 38% and 35% of NH3—N, respectively. All these media had the same capability to remove BOD, by microbial oxidation process. The research group found that compost and wood mulch had a tendency to increase the pH. They concluded that specific surface area, void space, permeability, and adsorption capacity might influence the removal efficiency.
Phosphorus removal from storm water is both precipitation and adsorption processes due to chemical reaction. As phosphorus has enormous effect on aquatic ecosystem, researchers have been trying to discover an economically feasible removal procedure. Some functionalized sorption media that can be used for phosphorus removal are sand rich with Fe, Ca or Mg, gravel, limestone, shale, light weight aggregates (LWA), zeolite (natural mineral or artificially produced alumino silicates), pelleted clay (along or in combination with soils), opaka (a siliceous sedimentary rock), pumice (natural porous mineral), wollastonite (a calcium metasilicate), fly ash, blast furnace slag (BFGS—a porous non-metallic co-product in iron industry), alum, goethite (a hydrous ferric oxide), hematite (a mineral form of iron(III) oxide), dolomite and calcite as described in Korkusuz, E. A., Beklioglu, M., and Demirer, G. N., Use of blast furnace granulated slag as a substrate in vertical flow reed beds; field application, Bioresourse Technology, 98, pp. 2089-2101, (2007).
DeBusk, T. A., Langston, M. A., Schwegler, B. R., and Davidson, S., 1997 describes an evaluation of filter media for treating storm water runoff, Proceedings of the fifth Biennial Storm water Research Conference, pp. 82-89 (1997) used sand with quartz, fresh organic peat soil, crushed lime rock (2.5 cm nominal size) and wollastonite (a mine containing calcium metasilicate plus ferrous metasilicate) to remove phosphorus, nickel and cadmium from storm water. They found that wallastonite had very good removal efficiency for their targeted contaminants. Wallastonite could remove about 87.8% P, 97.7% Cd and 80.3% Ni. On the other hand, limerock, peat and sand could remove 41.4%, 44%, and 41.4% P respectively. It was concluded that wallastonite is very effective in phosphorus removal because it contains calcium and ferrous ions. Calcium and ferrous ions can remove phosphorus by precipitation reaction or adsorption.
Hsieh and Davis (2003) found good total phosphorus (TOP) removal (about 41% to 48) by sand and concluded that it might happen due to simple adsorption or complex sorption/precipitation processes. They found that mulch was not a good candidate for TP removal. This research group concluded that TP removal was highly variable and it might be related to properties of sorption media used and flow pattern of nutrient laden water through the sorption media. Again, organic matter could also accelerate TP removal up to 93%.
Clark, S., Pitt, R., and Brown, D., Effect of anaerobiosis on filter media pollutant retention, Presented at the Engineering Foundation and the American Society of Civil Engineers Conference on Information & monitoring needs for evaluating the mitigation effects of BMPs, Snowmass, Colo. (2001) tried to remove contaminants in aerobic and anaerobic conditions from storm water runoff by using activated carbon, peat moss, compost and sand. They found good phosphorus removal efficiency by all four media in both conditions. They also found no desorption condition in their system for phosphorus. But they observed that sorption was better and leaching was lesser in aerobic condition for compost.
Forbes, M. G., Dickson, K. I., Saleh, F., Doyle, R. D., Hudak, P., and Waller W. T., Recovery and fractionation of phosphate retained by lightweight expanded shale and masonry sand used as media in subsurface flow treatment wetlands, Environmental Science & Technology, 39(12), pp. 4621-4627 (2005) used lightweight expanded shale and masonry sand for the removal of phosphorus. They summarized that sand is a poor candidate for retaining phosphorus and expanded shale has greater removal efficiency due to its larger surface area.
Researchers have used a variety of sorption media to remove nutrient, both nitrogen and phosphorus species, from storm water and wastewater. For removing nitrogen and phosphorus from storm water or wastewater, these filtration media can be further classified based on the derivation from: 1) plants or processed from components of plants; 2) sand and clay; 3) minerals; and 4) waste materials that may be recycled from the society.
Known prior art patents include U.S. Pat. No. 6,458,179 issued Oct. 1, 2002 for the use of shredded rubber as one material in a fertilizer; U.S. Pat. No. 5,823,711 issued Oct. 20, 1998 discloses use of scrap and shredded tires for improved drainage; U.S. Pat. No. 5,509,230 issued Apr. 23, 1996 discloses a lawn protecting method and elastic body for lawn protection used to minimize compaction under turf grasses; U.S. Pat. No. 5,014,462 issued May 14, 1991 discloses use of a soil amendment with rubber particles for increased porosity and reduced compaction; U.S. Pat. No. 6,969,469 issued Nov. 29, 2005 is directed toward a method of using waster tires as a filter media in a water treatment filtering device; and U.S. Pat. No. 6,214,229 issued on Apr. 10, 2001 discloses a treatment system for removing phosphorus from electric parts.